Printed circuit board and differential signaling structure

ABSTRACT

Provided is a system adopting a differential signaling system including a low frequency signaling line arranged to be adjacent to a pair of differential signaling lines in parallel to each other, for transmitting a signal having a frequency which is smaller than a frequency of a signal to be transmitted through the pair of differential signaling lines, in which a transmission end of the low frequency signaling line is connected to a ground pattern through a first capacitive element, and a reception end of the low frequency signaling line is connected to the ground pattern through a second capacitive element. Thus, it is possible to provide, easily and at a low cost, a differential signaling system in which a common mode noise is eliminated without increasing the number of pins.

BACKGROUND OF THE INVENTION

1. Field of the Invention The present invention relates to adifferential signaling structure adopting a differential signalingsystem, in which a radiation noise from an electronic instrument isreduced.

2. Related Background Art

In recent years, in a signal transmission between electronicinstruments, it is necessary to improve a data transmission rate so asto be compatible with a high-speed operation of the electricinstruments. To improve the data transmission rate, there are required ahigher frequency of a signal to be transmitted and a higher switchingspeed of the devices used for the signal transmission. To comply withthe higher frequency and the higher speed of the signal to betransmitted, it is necessary to take measures against radiation noises.For this reason, a differential signaling system has been used in placeof a conventional signaling system of a single-ended signaling system.In particular, a low voltage differential signaling (LVDS) system has agreat effect in reducing the radiation noises because a signal wave formthereof is a low-amplitude voltage, in addition to the effect ofcanceling magnetic fields generated by anti-phase currents flowingthrough a differential signal pair with other.

FIG. 8 is a schematic diagram showing a general circuit structureadopting the LVDS system. In FIG. 8, reference numeral 1000 denotes aprinted circuit board, reference numeral 100 denotes a transmission sidecircuit element, reference numeral 101 denotes a reception side circuitelement, and reference numeral 300 denotes a ground pattern. Referencenumeral 1001 denotes a printed wiring board, and the transmission sidecircuit element 100 and the reception side circuit element 101 aremounted on the printed wiring board 1001. Between the transmission sidecircuit element 100 and the reception side circuit element 101, adifferential signaling line 8 is arranged by providing signaling lines 1and 2 that have the same electrical characteristics, thereby performingdifferential signaling with a low-amplitude voltage.

A terminating resistor 3 having a value substantially equal to adifferential impedance of the differential signaling line is providedbetween in the vicinity of input terminals of the reception side circuitelement 101 and connected to the signaling lines 1 and 2. By providingthe terminating resistor 3, the entire anti-phase currents flowingthrough the signaling lines 1 and 2 are thermally consumed, therebymaking it possible to suppress the distortion of a wave form and thegeneration of radiation noises due to reflection. The signaling lines 1and 2 are arranged to be adjacent in substantially parallel to eachother and have the same length. As a result, the anti-phase currentsflowing through the signaling lines 1 and 2 generate magnetic fieldshaving substantially the same quantity in opposite directions to becancelled out, thereby making it possible to suppress generation of theradiation noises.

In FIG. 8, in addition to the differential signaling line 8 fortransmitting a high frequency signal, there are provided three lowfrequency signaling lines 4, 5 and 6, and a ground line 7. The lowfrequency signaling lines 4, 5, and 6 are connected to the transmissionside circuit elements 200, 202 and 204, respectively, and to thereception side circuit elements 201, 203 and 205, respectively, andtransmit a signal having an extremely small frequency as compared withthe differential signaling line 8. The signal transmitted through thelow frequency signaling lines 4, 5 and 6 have a small frequency, so thatthe radiation noise is not a problem even by adopting the transmissionsystem of the single-ended signaling system. Both ends of the groundpattern line 7 each are connected to a ground pattern 300, and theground line 7 constitutes return paths for the differential signalingline 8 and the low frequency signaling lines 4, 5 and 6.

The differential signaling system represented by the LVDS system iseffective in reducing the radiation noises due to the high frequencysignal. However, to comply with the higher frequency and the higherspeed of the signal, a standard for the radiation noises becomes morestringent year after year, so that the differential signaling system isnot sufficient enough to deal with the radiation noises.

Even when two signaling lines of the differential signaling line aredesigned to have completely the same electrical characteristics, anin-phase current component is generated in the differential signalingline due to a time lag within the transmission side circuit element, adifference between build up and build down characteristics thereof, andthe like. The differential signaling system is effective for ananti-phase signal, but not capable of suppressing the radiation noisesgenerated due to an in-phase signal. The radiation noise generated inthe differential signaling line due to the in-phase current component iscalled a common mode noise.

In a case of a circuit structure shown in FIG. 8, the in-phase currentcomponent flows through the differential signaling line 8, therebygenerating the common mode noise. With regard to the in-phase currentcomponent flowing from the transmission side circuit element 100 to thereception side circuit element 101, there is no path through which thein-phase current component flows past the reception side circuit element101. As a result, the in-phase current component returns to thetransmission side circuit element 100 through a stray capacitance andthe like of the printed wiring board while straying, whereby theradiation noises is generated.

Japanese Patent Application Laid-Open No. H11-205118 proposes anapplication of a center tap terminal circuit to a differential signalingsystem shown in FIG. 9. In FIG. 9, reference numerals 10 and 11 denoteresistors which are designed to have about a half value of adifferential impedance of a differential signaling line. The resistors10 and 11 are connected in series to signaling lines 1 and 2 between thesignaling lines 1 and 2 in the vicinity of input terminals of thereception side circuit element 101. Reference numeral 12 is a capacitorprovided between a connection point of the resistors 10 and 11 connectedin series and a ground pattern 300 to connect the connection point tothe ground pattern. An in-phase current component generated in thedifferential signaling lines 1 and 2 flows to the ground pattern 300through the resistors 10 and 11 having the same value, and the capacitor12. Then, the in-phase current component returns to a reception sidecircuit element 100 through a ground line 7, which is connected to theground pattern, as a return path. Thus, it is possible to suppress theradiation noise.

Japanese Patent Application Laid-Open No. 2001-007458 proposes anapplication of a center tap terminal circuit to a differential signalingsystem shown in FIG. 10. In FIG. 10, the radiation noise is reduced bydevising an arrangement of in-phase current components and ground lineswhich become return paths for the current components. In FIG. 10,reference numerals 13 and 14 denote ground lines that are newly providedto be adjacent to and in substantially parallel to signaling lines 1 and2, and that are each connected to the ground pattern 300. With thisstructure, the in-phase current components generated in the signalinglines 1 and 2 flow through a center tap terminal constituted of theresistors 10 and 11 and the capacitor 12, and then returns to thereception side circuit element 100 through the two ground lines 13 and14. In this case, the magnetic fields generated by the in-phase currentcomponent flowing through the signaling lines 1 and 2 and the magneticfields generated by the return current flowing through the ground lines13 and 14 cancel out each other in the vicinity of a pole, therebymaking it possible to reduce the radiation noise.

However, in the case of the differential signaling system shown in FIG.9, when the ground line 7 serving as the return path for the in-phasecurrent is apart from the signaling lines 1 and 2, a current loopbecomes large. As a result, the effect of reducing the radiation noiseis not obtained sufficiently.

In addition, in the case of the differential signaling system shown inFIG. 10, it is necessary to add two ground lines each time of adding apair of differential signaling lines. This increases the number ofconnector pins and cable cores. Further, this causes an increase inpacking density and contour size of a printed wiring board on which theconnector is mounted, an increase in cross-sectional area of the cable,and the like, thereby increasing a manufacturing cost of a circuit, andpreventing an electric instrument from being small-sized. A higher speedsystem requires more lines which need to be switched to the differentialsignaling system, thereby making the problems more serious.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide, easily andat a low cost, a structure for reducing radiation noise adopting adifferential signaling system, in which the problem of the radiationnoise inherent to the differential signaling system can be solvedwithout increasing the number of pins, in a signal transmission betweenelectric instruments, in a case where the differential signaling systemis carried out in order to reduce the radiation noise in associationwith an increase in frequency and device switching speed for improvementof a data transmission rate.

To solve the above-mentioned problems, the present invention provides adifferential signaling structure including: a pair of differentialsignaling lines provided between a transmission side circuit element anda reception side circuit element; and a low frequency signaling linearranged to be adjacent to and in parallel to the differential signalinglines, for transmitting a signal having a frequency smaller than afrequency of a signal to be transmitted through the differentialsignaling lines, in which a transmission end of the low frequencysignaling line is connected to a ground pattern through a firstcapacitive element, and a reception end of the low frequency signalingline is connected to the ground pattern through a second capacitiveelement.

Further, the present invention provides a differential signalingstructure including: a pair of differential signaling lines providedbetween the transmission side circuit element and the reception sidecircuit element; and a low frequency signaling line arranged to beadjacent to and in parallel to the differential signaling lines, fortransmitting a signal having a frequency smaller than a frequency of asignal to be transmitted through the differential signaling lines, inwhich, in the vicinity of an input terminal of the reception sidecircuit element of the pair of differential signaling lines, tworesistors are arranged which are connected in series to the pair ofdifferential signaling lines, and which have a resistance value that isabout a half of a resistance value matching a differential impedance;the second capacitive element is connected between a connection point ofthe two resistors connected in series with each other and thetransmission end of the low frequency signaling line; and the receptionend of the low frequency signaling line is connected to the groundpattern through a third capacitive element.

Further, the present invention provides a printed circuit boardincluding: a pair of differential signaling lines provided between atransmission side circuit element and a reception side circuit element;and a low frequency signaling line arranged to be adjacent to and inparallel to the differential signaling lines, for transmitting a signalhaving a frequency smaller than a frequency of the signal to betransmitted through the differential signaling lines, wherein atransmission end of the low frequency signaling line is connected to theground pattern through the first capacitive element, and a reception endof the low frequency signaling line is connected to the ground patternthrough the second capacitive element.

The above and other objects of the invention will become more apparentfrom the following description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a circuit structure adopting adifferential signaling system according to a first embodiment of thepresent invention;

FIG. 2 is a schematic diagram showing the circuit structure adopting thedifferential signaling system according to the first embodiment of thepresent invention;

FIG. 3 is a schematic diagram showing a circuit structure adopting adifferential signaling system according to a second embodiment of thepresent invention;

FIG. 4 is a schematic diagram showing a circuit structure adopting adifferential signaling system according to a third embodiment of thepresent invention;

FIG. 5 is a schematic diagram showing another embodiment of the presentinvention;

FIG. 6 is a schematic diagram showing still another embodiment of thepresent invention;

FIG. 7 is a graph showing experimental results according to the presentinvention;

FIG. 8 is a schematic diagram showing a conventional circuit structure;

FIG. 9 is a schematic diagram showing the conventional circuitstructure; and

FIG. 10 is a schematic diagram showing the conventional circuitstructure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described with reference tothe drawings.

First Embodiment

FIG. 1 is a schematic diagram showing a circuit structure according to afirst embodiment of the present invention. It should be noted that thesame reference numerals are given to the members which are the same asthose of FIGS. 8, 9 and 10. In this embodiment, only the parts differentfrom prior arts will be described.

In FIG. 1, reference numeral 15 denotes a capacitor which is provided soas to connect the vicinity of an output terminal of a transmission sidecircuit element 200 of a low frequency signaling line 4 and a groundpattern 300 to each other, and reference numeral 16 denotes a capacitorwhich is provided so as to connect the vicinity of an input terminal ofa reception side circuit element 201 of the low frequency signaling line4 and the ground pattern 300 to each other. Referring to FIG. 10 forcomparison, a ground line 14 arranged to be adjacent to one side of asignaling line 2 constituting a differential signaling line is removed,and the low frequency signaling line 4 is arranged in its position, thatis, to be adjacent to and in substantially parallel to the signalingline 2. With this structure, an in-phase current component flowingthrough differential signaling lines 1 and 2 reaches the ground pattern300 through resistors 10 and 11 and a capacitor 12. Further, thein-phase current component returns to a reception side circuit element100 through a ground line 13, and at the same time, returns to thereception side circuit element 100 through the capacitor 16, the lowfrequency signaling line 4, and the capacitor 15. In this case, themagnetic field generated by the in-phase current component flowingthrough the differential signaling lines 1 and 2 and the magnetic fieldgenerated by a return current flowing through the ground line 13 and thelow frequency signaling line 4 cancel out each other in the vicinity ofa pole, thereby making it possible to suppress generation of radiationnoise.

At this time, through the low frequency signaling line 4, a lowfrequency signal is transmitted from the transmission side circuitelement 200 to the reception side circuit element 201. As a result, itis necessary that the return current is caused to flow in a state wherethe return current does not substantially affect the low frequencysignal. In the low frequency signal, when a voltage of the signal to betransmitted from the transmission side circuit element 200 is suppressedto an attenuation factor of 10% or less at the reception side circuitelement 201, a failure in signal transmission is not caused.

FIG. 2 is a schematic diagram showing a circuit structure of the lowfrequency signaling line 4. In FIG. 2, reference symbol Vs denotes asignal source of a transmission side circuit element 200 of a lowfrequency signal, and reference symbol Zo denotes an output impedance ofa transmission side circuit element 200 of a low frequency signal. Apoint A denotes a signal reception terminal of the reception sidecircuit element 201. A voltage generated when a voltage of the signalsource Vs is received at the signal reception terminal A is determinedby an input impedance Zi of the reception side circuit element 201 andby an impedance caused when capacitors C1 and C2 are connected inparallel with each other. The impedance is reduced when the capacitorsC1 and C2 are connected in parallel with each other, so that a voltageamplitude is reduced as compared with a case of an input impedance Zi inabsence of the capacitors C1 and C2.

Assuming that a minimum pulse width of the signal to be transmittedthrough the low frequency signaling line 4 is set to imin, a frequencyof the signal to be transmitted is represented as the followingreciprocal which is a reciprocal of twice the minimum pulse width tmin:1/(2×τmin)  Formula (1).Accordingly, within a bandwidth of equal to or less than the frequencyindicated by Formula (1), it is sufficient that the attenuation factorof the voltage is set to equal to or less than 10%.

In general, in a CMOS-IC, the output impedance Zo is extremely small ascompared with the input impedance Zi of the reception side circuitelement 201 of a low frequency signal, and is represented as thefollowing formula:Zo<<Zi  Formula (2).

Thus, in order to suppress the attenuation factor of the voltageamplitude to 10% or less, when it is assumed that the impedance of aparallel circuit including the capacitors C1 and C2 is Zc, it issufficient that Zc is set to a value ten or more times Zi as representedby the following formula.Zc>10×Zi  Formula (3).

This is because a total impedance caused when Zi is connected inparallel with Zc is represented as the following formula:(Zc×Zi)/(Zc+Zi)=(10/11)×Zi  Formula (4)and the attenuation factor of the impedance becomes (10/11), in otherwords, 10% or less. The impedance Zc of the parallel circuit includingthe capacitors C1 and C2 at the frequency of f is given by the followingformula:Zc=1/(2π×f×(C1+C2))  Formula (5).Then, f is obtained by substituting the frequency given by Formula (2)as represented by the following formula:Zc=τmin/(π×(C1+C2))  Formula (6).When Formula (6) is substituted for Formula (3), the following formulacan be obtained.C1+C2<((τmin)/(10×π×Zi))  Formula (7).

In other words, the total value of the C1 and C2 makes it a conditionthat Formula (7) is satisfied. Formula (7) determines a necessarycondition for receiving the signal having the minimum pulse width τminat the reception side circuit element 201 without a failure.

By setting such the condition, the attenuation factor of the wave formamplitude of the low frequency signal is suppressed to be reduced by 10%at a maximum, thereby making it possible to achieve reduction inradiation noise without causing any failures in operation.

Second Embodiment

FIG. 3 is a schematic diagram showing a circuit structure according to asecond embodiment of the present invention. It should be noted that thesame reference numerals are given to the members which are the same aswith those of FIG. 1 representing the first embodiment. In thisembodiment, only the parts different from the first embodiment will bedescribed.

In FIG. 3, reference numeral 17 denotes a capacitor which is provided soas to connect the vicinity of an output terminal of a transmission sidecircuit element 202 of a low frequency signaling line 5 and a groundpattern 300, and reference numeral 18 denotes a capacitor which isprovided so as to connect the vicinity of an input terminal of areception side circuit element 203 of the low frequency signaling line 5and the ground pattern 300. The low frequency signaling line 5 isarranged to be adjacent to and in substantially parallel to adifferential signaling line 1. In this case, the low frequency signalinglines 4 and 5 for carrying out the low frequency signaling have the sameelectrical characteristics, and are arranged such that each distancefrom the lines 4 and 5 to differential signaling lines 1 and 2 is set tobe equal. In addition, a capacitor 15 and the capacitor 17 have the samecapacitance value, and a capacitor 16 and the capacitor 18 also have thesame capacitance value. With the above-mentioned structure, a returncurrent of an in-phase current passing through the differentialsignaling lines 1 and 2 flows through each of the low frequencysignaling lines 4 and 5 at the same level. As a result, the magneticfield generated by the in-phase current and the magnetic field generatedby the return current cancel out each other in contrast to each otherand in a balanced manner, thereby making it possible to further reducethe radiation noise.

It should be noted that each capacitance value of the capacitors 15, 16,17 and 18 can be obtained in the same manner as in the first embodiment.

Third Embodiment

FIG. 4 is a schematic diagram showing a circuit structure according to athird embodiment of the present invention. It should be noted that thesame reference numerals are given to the members which are the same asthose of FIG. 3 representing the second embodiment. In this embodiment,only the parts different from the second embodiment will be described.

In FIG. 4, reference numeral 19 denotes a capacitor which is provided soas to connect the vicinity of an output terminal of a reception sidecircuit element 201 of a low frequency signaling line 4 and a midpointbetween center tap terminating resistors 10 and 11, and referencenumeral 20 denotes a capacitor which is provided so as to connect thevicinity of an input terminal of a reception side circuit element 203 ofa low frequency signaling line 5 and the midpoint between the center tapterminating resistors 10 and 11. The low frequency signaling lines 4 and5 are arranged to be adjacent to and in substantially parallel todifferential signaling lines 1 and 2. A return current of an in-phasecurrent flowing through the differential signaling lines 1 and 2 isdivided in each direction of the capacitors 19 and 20 at the midpointbetween the center tap terminating resistors 10 and 11. Further, thereturn current flows through the low frequency signaling lines 4 and 5to a ground pattern 300 through capacitors 15 and 17, respectively, andthen returns to a transmission side circuit element 100 of adifferential signal.

With this structure, as compared with the second embodiment, even whenthe number of capacitors is reduced by one, the same effect of reducingthe radiation noise can be obtained, thereby making it possible toreduce packaging area and the manufacturing cost.

It should be noted that each capacitance value of the capacitors 15, 17,19 and 20 can be obtained in the same manner as in the first embodiment.

As the transmission side circuit elements 100, 200, 202 and 204according to the first, second and third embodiments, various ICs may beused. In a similar manner, as the reception side circuit elements 101,201, 203 and 205 according to the first, second and third embodiments,various ICs may be used.

Further, as shown in FIG. 5, the transmission side circuit elements 100,200, 202 and 204 and a ground pattern 301 may be set as differentterminals of the same IC package 2000, and the reception side circuitelements 101, 201, 203 and 205 and a ground pattern 302 may be also setas different terminals of the same IC package 2001.

Further, as shown in FIG. 6, the transmission side circuit elements 100,200, 202 and 204 may be set as different terminals of the same connector3000, and the reception side circuit elements 101, 201, 203 and 205 maybe also set as different terminals of the same connector 3001. In thiscase, signaling lines 1 to 6 are arranged within a cable 3002.

EXPERIMENTAL EXAMPLE

In the differential signaling system shown in FIG. 3 according to thesecond embodiment, a strength of a generated electric field was obtainedby simulation.

A structure in which experimental results shown in FIG. 7 were obtainedwill be described. In each case of a differential signaling system shownin FIGS. 3, 9 and 10, transmission side circuit elements 100, 200, 202and 204 were arranged on a left side of a printed circuit board 1000,and reception side circuit elements 101, 201, 203 and 205 were arrangedon a right side of the printed circuit board 1000. Signaling lines 1, 2,3, 4, 5, 6, 7 and 13 each have a wire diameter of 0.1 mm and a length of50 mm. The signaling lines were arranged to be parallel with each otherat an interval of 2 mm. Terminating resistors 10 and 11 were set to 50Ω, and a capacitor 12 was set to 0.1 μF, thereby constituting a centertap terminal circuit. Further, the electric field strength obtained in acase where capacitors 15, 16, 17 and 18 were set to 50 pF The strengthof the electric field generated in this case is represented as thesymbol “o” as shown in FIG. 7. It should be noted that the simulationresult indicates the electric field strength obtained when an object tobe measured was arranged at a level of 80 cm from a ground patternsurface by the 3m method. Further, the electric field strength obtainedin a case where capacitors 15 and 17 were set to 10 pF, and capacitors16 and 18 were set to 90 μF is indicated as the symbol “x” as shown inFIG. 7. Further, for comparison, the electric field strength in thedifferential signaling system shown in FIG. 9 is represented as thesymbol “*”, and the electric field strength obtained in a case where aground line was arranged at both adjacent sides of the differentialsignaling line shown in FIG. 10 is represented as the symbol “Δ”.

As apparent from FIG. 7, the electric field strengths of “o” and “x”indicating the differential signaling system according to the presentinvention are lowered by 10 dB or more as compared with the electricfield strength of “*” indicating the conventional differential signalingsystem shown in FIG. 9, and radiation noise is suppressed to a largeextent. In addition, it is apparent that the electric field strengths of“o” and “x” have substantially the same value as compared with theelectric field strength of “Δ” indicating the conventional differentialsignaling system shown in FIG. 10.

Therefore, it is found that it is possible to obtain the same effect ofreducing the radiation noise by using an extremely simple methodaccording to the present invention, as compared with the differentialsignaling system shown in FIG. 10.

This application claims priorities from Japanese Patent Application Nos.2005-209881 filed on Jul. 20, 2005, and 2006-186912 filed on Jul. 6,2006, which are hereby incorporated by reference herein.

1. A printed circuit board, comprising: a printed wiring board; atransmission side circuit element mounted on the printed wiring board; areception side circuit element mounted on the printed wiring board; apair of differential signaling lines provided between the transmissionside circuit element and the reception side circuit element; and asignaling line arranged to be adjacent to and in parallel to the pair ofdifferential signaling lines, for transmitting a signal having afrequency smaller than a frequency of a signal to be transmitted throughthe pair of differential signaling lines, wherein a transmission end ofthe signaling line is connected to a ground pattern of the printedwiring board through a first capacitive element; and a reception end ofthe signaling line is connected to the ground pattern of the printedwiring board through a second capacitive element.
 2. A printed circuitboard according to claim 1, further comprising a center tap terminal inthe vicinity of an input terminal of the reception side circuit elementof the pair of differential signaling lines, the center tap terminalincluding: two resistors connected in series with the pair ofdifferential signaling lines, having a resistance value which is about ahalf of a resistance value matching a differential impedance; and athird capacitive element having two end, one end being connected to aconnection point of the two resistors connected in series with eachother, and the other end being connected to the ground pattern.
 3. Aprinted circuit board according to claim 1, wherein an impedanceobtained by adding capacitance values of the first capacitive elementand the second capacitive element is equal to or more than ten times aninput impedance of a reception end of the signaling line at a frequencycalculated as a reciprocal of twice a minimum pulse width of a signal tobe transmitted through the signaling line.
 4. A printed circuit boardaccording to claim 1, wherein the pair of differential signaling linesare arranged between the signaling line and a ground line.
 5. A printedcircuit board according to claim 1, wherein the signaling line isarranged to be adjacent to both sides of the pair of differentialsignaling lines, and wherein the signaling line has an impedance equalto an impedance of a ground pattern.
 6. A printed circuit board,comprising: a printed wiring board; a transmission side circuit elementmounted on the printed wiring board; a reception side circuit elementmounted on the printed wiring board; a pair of differential signalinglines provided between the transmission side circuit element and thereception side circuit element; and a signaling line arranged to beadjacent to and in parallel to the pair of differential signaling lines,for transmitting a signal having a frequency smaller than a frequency ofa signal to be transmitted through the pair of differential signalinglines, wherein in a vicinity of an input terminal of the reception sidecircuit element of the pair of differential signaling lines, tworesistors are arranged, which are connected in series with the pair ofdifferential signaling lines, and which have a resistance value that isabout a half of a resistance value matching a differential impedance; asecond capacitive element is connected between a connection point of thetwo resistors connected in series with each other and a transmission endof the signaling line; and a reception end of the signaling line isconnected to a ground pattern through a third capacitive element.
 7. Aprinted circuit board, comprising: a printed wiring board; atransmission side circuit element mounted on the printed wiring board; areception side circuit element mounted on the printed wiring board; apair of differential signaling lines provided between the transmissionside circuit element and the reception side circuit element; and asignaling line arranged to be adjacent to and in parallel to the pair ofdifferential signaling lines, for transmitting a signal having afrequency smaller than a frequency of a signal to be transmitted throughthe pair of differential signaling lines, wherein a transmission end ofthe signaling line is connected to a ground pattern through a firstcapacitive element, and a reception end of the signaling line isconnected to the ground pattern through a second capacitive element; anda common mode noise to be transmitted through the pair of differentialsignaling lines is transmitted via the first capacitive element, thesignaling line, and the second capacitive element.
 8. A differentialsignaling structure, comprising: a transmission side circuit element; areception side circuit element; a pair of differential signaling linesprovided between the transmission side circuit element and the receptionside circuit element; and a signaling line arranged to be adjacent toand in parallel to the pair of differential signaling lines, fortransmitting a signal having a frequency smaller than a frequency of asignal to be transmitted through the pair of differential signalinglines, wherein a transmission end of the signaling line is connected toa ground pattern through a first capacitive element, and a reception endof the signaling line is connected to the ground pattern through asecond capacitive element.
 9. A differential signaling structureaccording to claim 8, further comprising a center tap terminal in thevicinity of an input terminal of the reception side circuit element ofthe pair of differential signaling lines, the center tap terminalincluding: two resistors connected in series with the pair ofdifferential signaling lines, and having a resistance value which isabout a half of a resistance value matching a differential impedance;and a third capacitive element having two ends, one end being connectedto a connection point of the two resistors connected in series with eachother, and the other end being connected to the ground pattern.